When I first started fly-fishing, my friend and mentor taught me about his fishing notebook. During my first trip to Wyoming, Yellowstone, and Montana, we drifted the Snake and Yellowstone Rivers and waded through Soda Butte and Slough Creeks. The night before each different water, my friend would pull out his fishing notebook and review his notes from his last few experiences on that water.
He would read about the bugs he saw, which flies seems to work best, and other important variables such as water flows and temperature. He eventually went on to become a certified guide.
Witnessing first-hand how much that data helped him, I started to keep my own log to chart my journal and learn each experience. Ever water is unique.
Conditions change each season, as do an angler's tactics. The information gained from each experience can be invaluable.